[clang] [-Wunsafe-buffer-usage] Fix false warnings when const sized array is safely accessed (array [idx %const]) (PR #140113)

[Malavika Samak via cfe-commits]

https://github.com/malavikasamak updated https://github.com/llvm/llvm-project/pull/140113

>From c6d8a58a8468fb3e6bcbe7d5935e2aa033745d99 Mon Sep 17 00:00:00 2001
From: MalavikaSamak <malavika2 at apple.com>
Date: Thu, 15 May 2025 10:46:01 -0700
Subject: [PATCH 1/2] [-Wunsafe-buffer-usage] Fix false warnings when const
 array is safely accessed array [idx %const]

The -Wunsafe-buffer-usage analysis currently warns when a const sized array is safely accessed,
with an index that is bound by the remainder operator. The false alarm is now suppressed.

Example:

int circular_buffer(int array[10], uint idx) {
   return array[idx %10]; // Safe operation
}

rdar://148443453
---
 clang/lib/Analysis/UnsafeBufferUsage.cpp        | 17 ++++++++++++++++-
 .../SemaCXX/warn-unsafe-buffer-usage-array.cpp  | 10 +++++++++-
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Analysis/UnsafeBufferUsage.cpp b/clang/lib/Analysis/UnsafeBufferUsage.cpp
index 5b72382ca9772..2f669637a2731 100644
--- a/clang/lib/Analysis/UnsafeBufferUsage.cpp
+++ b/clang/lib/Analysis/UnsafeBufferUsage.cpp
@@ -600,12 +600,27 @@ static bool isSafeArraySubscript(const ArraySubscriptExpr &Node,
   } else if (const auto *BE = dyn_cast<BinaryOperator>(IndexExpr)) {
     // For an integer expression `e` and an integer constant `n`, `e & n` and
     // `n & e` are bounded by `n`:
-    if (BE->getOpcode() != BO_And)
+    if (BE->getOpcode() != BO_And && BE->getOpcode() != BO_Rem)
       return false;
 
     const Expr *LHS = BE->getLHS();
     const Expr *RHS = BE->getRHS();
 
+    if(BE->getOpcode() == BO_Rem) {
+      // If n is a negative number, then n % const can be greater than const
+      if(!LHS->getType()->isUnsignedIntegerType()) {
+        return false;
+      }
+
+      if(!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
+        llvm::APSInt result = EVResult.Val.getInt();
+        if (result.isNonNegative() && result.getLimitedValue() <= limit)
+          return true;
+      }
+
+      return false;
+    }
+
     if ((!LHS->isValueDependent() &&
          LHS->EvaluateAsInt(EVResult, Ctx)) || // case: `n & e`
         (!RHS->isValueDependent() &&
diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
index 3233999ea8ea2..b2358bb9f6be3 100644
--- a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
@@ -35,7 +35,15 @@ void constant_idx_safe0(unsigned idx) {
   buffer[0] = 0;
 }
 
-int array[10]; // expected-warning {{'array' is an unsafe buffer that does not perform bounds checks}}
+int array[10]; // expected-warning 2{{'array' is an unsafe buffer that does not perform bounds checks}}
+
+void circular_access_safe(unsigned idx) {
+  array[idx % 10];
+}
+
+void circular_access_unsafe(int idx) {
+  array[idx % 10]; // expected-note {{used in buffer access here}}
+}
 
 void masked_idx1(unsigned long long idx, Foo f) {
   // Bitwise and operation

>From 334b062ccf7e93f93653a6ef989e36b3524efdc8 Mon Sep 17 00:00:00 2001
From: MalavikaSamak <malavika2 at apple.com>
Date: Thu, 15 May 2025 11:57:51 -0700
Subject: [PATCH 2/2] Fixing the formatting issue and added tests.

---
 clang/lib/Analysis/UnsafeBufferUsage.cpp             |  6 +++---
 .../test/SemaCXX/warn-unsafe-buffer-usage-array.cpp  | 12 +++++++++---
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/clang/lib/Analysis/UnsafeBufferUsage.cpp b/clang/lib/Analysis/UnsafeBufferUsage.cpp
index 2f669637a2731..ec648e1a17af9 100644
--- a/clang/lib/Analysis/UnsafeBufferUsage.cpp
+++ b/clang/lib/Analysis/UnsafeBufferUsage.cpp
@@ -606,13 +606,13 @@ static bool isSafeArraySubscript(const ArraySubscriptExpr &Node,
     const Expr *LHS = BE->getLHS();
     const Expr *RHS = BE->getRHS();
 
-    if(BE->getOpcode() == BO_Rem) {
+    if (BE->getOpcode() == BO_Rem) {
       // If n is a negative number, then n % const can be greater than const
-      if(!LHS->getType()->isUnsignedIntegerType()) {
+      if (!LHS->getType()->isUnsignedIntegerType()) {
         return false;
       }
 
-      if(!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
+      if (!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
         llvm::APSInt result = EVResult.Val.getInt();
         if (result.isNonNegative() && result.getLimitedValue() <= limit)
           return true;
diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
index b2358bb9f6be3..00daa28b433eb 100644
--- a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
@@ -35,13 +35,19 @@ void constant_idx_safe0(unsigned idx) {
   buffer[0] = 0;
 }
 
-int array[10]; // expected-warning 2{{'array' is an unsafe buffer that does not perform bounds checks}}
+int array[10]; // expected-warning 3{{'array' is an unsafe buffer that does not perform bounds checks}}
 
-void circular_access_safe(unsigned idx) {
+void circular_access_unsigned(unsigned idx) {
   array[idx % 10];
+  array[idx % 11]; // expected-note {{used in buffer access here}}
+  array[(idx + 3) % 10];
+  array[(--idx) % 8];
+  array[idx & 9 % 10];
+  array[9 & idx % 11];
+  array [12 % 10];
 }
 
-void circular_access_unsafe(int idx) {
+void circular_access_signed(int idx) {
   array[idx % 10]; // expected-note {{used in buffer access here}}
 }